1. Field of the Invention
The invention resides in the field of continuous processes and apparatus for making supported polymeric sheets including ion exchange membranes from polymerizable liquid precursors and continuous foraminous, solid substrates such as fabrics, paper and microporous diaphragms.
2. Description of the Prior Art
During the last four decades several million square feet of ion exchange membrane have been made from freely flowing solutions of functional or functionizable monomers, crosslinking monomers and low molecular weight, non-polymerizing diluents for said monomers. (See e.g. U.S. Pat. No. 2,730,768). The diluents are also characterized by being good swelling agents for the polymers eventually resulting from such monomers and by being readily leached out of such polymers. Almost all of such membranes have been reinforced with substrates such as woven or non-woven fabrics, paper or microporous diaphragms. The process generally comprised interleaving the substrates and glass plates in a pool of such solution in shallow trays, removing excess solution, polymerizing the monomers by heating the array of plates and substrates and subsequently disassembling the array to retrieve the polymer impregnated substrates. The glass plates were generally recycled. In some cases the polymer impregnated substrates were useful as ion exchange membranes after simple leaching in water (or other solvents and then water). In other cases functional monomers had first to be converted to suitable ion exchange groups (e.g. polyvinyl benzyl chloride-co-divinyl benzene to poly (N-benzyl-N,N,N-trimethyl ammonium chloride)-co-divinyl benzene). In yet other cases ion exchange groups had to be added to functionizable moieties (e.g. sulfonic acid groups to polystyrene-co-divinyl benzene).
Such process, as is, is inherently labor intensive and difficult to automate or robotize.
Processes for continuous polymerization are known. For example, Japanese Laid Open Patent 50-103585 describes method and apparatus for continuous polymerization in the absence of substrates in which both edges of a single continuous film are brought into contact in the longitudinal direction continuously to form a roughly circular tube. An aqueous solution of non-crosslinking monomers polymerizable to water soluble polymers and an aqueous solution of polymerization agent are supplied to the resulting tube and the polymerization of the monomers is carried out while the tube is moved by an endless belt. The tube is subsequently opened and the water soluble gel polymer recovered. The tube moved at 2 to 2.6 ft/hr. and the polymerization time was 4 to 5 hours. No support substrate is used.
European Patent Application 0 197 423 describes method and apparatus for continuous photopolymerization of vinyl and related monomers in aqueous solution in the absence of support substrates. The solution is deposited on a horizontal, endless, moving metal belt with continuous belt-like dams at the edges. The solution is covered with film such as nylon or polyester having a low permeability to oxygen and a high permeability to ultraviolet light.
U.S. Pat. No. 2,590,186 describes method and apparatus for forming a continuous, solid film from a solution or emulsion of an organic, high polymer, comprising advancing substantially vertically two individual continuous sheets together through a pair of horizontal squeeze rolls while feeding a solution or dispersion of the polymer between the converging inner surfaces of the sheets. At least one of the sheets is absorbent of the solvent or dispersant in the solution or dispersion resp. The solution or dispersion has a viscosity greater than 10 poise. No polymerization takes place. Dispersant or solvent (e.g. water) is absorbed from the dispersion or solution resp. by one or both of the sheets until the dispersant or solvent content of the latter is such that it behaves as a solid. The ultimate product can be one of the absorbent sheets coated with polymer. The thickness of polymer is about 0.1 to about 0.4 mils. The lateral edges of the sheets are not at least temporarily sealed together at a distance above the squeeze rolls to make a pocket capable of holding free flowing liquid. The sandwich consists of the two sheets and therebetween the solution or emulsion of high polymer.
U.S. Pat. No. 3,732,193 describes a thin film polymerization method for making water soluble polymers and copolymers of water-soluble, ethylenic-unsaturated, non-crosslinking monomers comprising casting an aqueous monomer solution in the presence of a free radial initiator on an endless belt to form a thin film, then passing the film through a heated tunnel (having an inert gas blanket) to polymerize the aqueous monomer in the form of a thin film and finally drying the film. A foraminous substrate for the non-crosslinked, water-soluble polymer is not used.
U.S. Pat. No. 3,912,607 describes a process for making non-crosslinked, thermoplastic water-soluble acrylic polymers or copolymers by depositing a concentrated aqueous solution of the (non-crosslinking) monomers and a photo polymerization initiator on a non-foraminous non-absorbent, horizontal endless belt as a continuous layer, illuminating the layer while controlling the temperature of the layer by cooling.
U.S. Pat. No. 4,324,606 describes a process and apparatus for thermally laminating two or more preformed, non-crosslinked, thermoplastic polyperfluoro-cation exchange ionomer films at least one to each side of a woven fabric utilizing two sets of flexible endless belts to transport such laminate into and out of the thermal laminating zone, one belt of each set having a series of perforations along its entire length, each set of belts extending beyond the adjacent edge of the laminate. The fabric is wider than the films to permit air to be pumped out from between the films.
U.S. Pat. No. 4,372,800 describes a continuous process for producing reinforced resin laminates comprising the steps of impregnating a plurality of fibrous substrates with a liquid resin which is free of volatile solvent and capable of curing without generating liquid and gaseous by-products and then laminating the plurality of the resin-impregnated substrates into a unitary member. The unitary member is sandwiched by a pair of horizontal squeeze rolls having the plane of their axes in a vertical direction and between a pair of horizontal covering sheets and cured between the sheets without applying appreciable pressure. The process suffers from the problem of controlling the amount of liquid resin held on each substrate to be neither more nor less than that required by the sandwiched unitary member. U.S. Pat. No. 4,372,800 acknowledges that "insufficient resin contents result in poor quality of the product, while excessive resin contents often cause various disadvantages such as spilling of the liquid resin out from the edges of the uncured laminate and the like" (col. 20 line 27 et. seq.). Further the thin, impregnated sheets offer a large area to the air before the sandwiching step, permitting the sorption of oxygen, the latter interferring with subsequent free radical curing steps. In addition the impregnated substrates are directed by other rollers from the impregnating means to the sandwiching squeeze rolls. Such directing rollers are also a mechanism for sorbing oxygen into the liquid resin, tend to build up tacky resin and can only be described as operationally messy and unsatisfactory. These problems are all solved in the present invention by first forming a sandwich of substrate(s) and covering sheets, said sandwich being substantially vertically oriented (e.g. deviating from the vertical by not more than about 45 degrees), sealing at least temporarily the edges of the covering sheets to each other to form a pocket above substantially horizontally oriented squeeze rolls having the plane of their axes substantially horizontal and feeding the polymerizable liquid to the pocket in the region thereof where the covering sheets converge between the squeeze rolls. In this way the sandwich of substrate(s) and covering sheets takes exactly that amount of liquid which is required by the wettable volume between the covering sheets, no more or no less and large areas of impregnated substrate are not exposed to air. The wettable volume between the covering sheets in the present invention is of course determined by the gap between the squeeze rolls, the thickness of the covering sheets (films, foils), the total thickness of the support substrate(s) and the wettable void volume of the latter as will be more fully described below.